1. Field of the Invention
Many solid state power supplies require forced air cooling in order to meet desired operational levels. To provide such cooling, it is common to use DC fans which include a rotation sensor providing an electrical output signal that drives an open collector transistor on and off as an indication when the fan is operating. With such open collector circuits, when the rotation of the fan is stopped, the output of the output transistor may assume either a high, for example 5 volt, or low, for example 0.5 volt, state depending upon the reference voltage value and the position of the rotor of the fan when it stops. Since most conventional sensing circuits use active logic to detect a missing pulse, it is difficult with conventional circuits to sense when such a DC fan has stopped, in order to provide an error or fault indication.
2. Description of the Prior Art
Various sensing circuits exist in the prior art to detect missing pulse signals and/or to detect error signals in response to the occurrence of a predetermined condition in semiconductor circuits. However, Applicant is not aware of a circuit which converts pulses of a pulse train to a DC level that can be compared to a reference, with the circuit insuring that a steady state voltage level exists that will decay to a constant lower value when the pulse train is interrupted, regardless of whether the output of the pulse train is stopped in a high or a low state.